The present invention is in the general field of diagnostic imaging agents, and is particularly directed to specific ultrasound contrast agent dosage formulations that provide enhanced images and images of long duration.
When using ultrasound to obtain an image of the internal organs and structures of a human or animal, ultrasound waves, waves of sound energy at a frequency above that discernable by the human ear, are reflected as they pass through the body. Different types of body tissue reflect the ultrasound waves differently and the reflections that are produced by the ultrasound waves reflecting off different internal structures are detected and converted electronically into a visual display.
For some medical conditions, obtaining a useful image of the organ or structure of interest is especially difficult because the details of the structure are not adequately discernible from the surrounding tissue in an ultrasound image produced by the reflection of ultrasound waves absent a contrast-enhancing agent. Detection and observation of certain physiological and pathological conditions may be substantially improved by enhancing the contrast in an ultrasound image by administering an ultrasound contrast agent to an organ or other structure of interest. In other cases, detection of the movement of the ultrasound contrast agent itself is particularly important. For example, a distinct blood flow pattern that is known to result from particular cardiovascular abnormalities may only be discernible by administering the ultrasound contrast agent to the bloodstream and observing either blood flow or blood volume.
Materials that are useful as ultrasound contrast agents operate by having an effect on ultrasound waves as they pass through the body and are reflected to create the image from which a medical diagnosis is made. Different types of substances affect ultrasound waves in different ways and to varying degrees. Moreover, certain of the effects caused by contrast-enhancing agents are more readily measured and observed than others. In selecting an ideal composition for an ultrasound contrast agent, one would prefer the substance that has the most dramatic effect on the ultrasound wave as it passes through the body. Also, the effect on the ultrasound wave should be easily measured. Gases are the preferred media for use as ultrasound contrast agents. The gas must be stabilized prior to usage as either surfactant stabilized bubbles or by encapsulating in liposomes or microparticles. There are three main contrast-enhancing effects which can be seen in an ultrasound image: backscatter, beam attenuation, and speed of sound differential.
A variety of natural and synthetic polymers have been used to encapsulate ultrasound contrast agents, such as air, in an effort to make an ultrasound contrast agent that lasts longer following administration. Schneider et al., Invest. Radiol., Vol. 27, pp. 134-139 (1992) describes three micron, air-filled, synthetic, polymer particles. These particles were reported to be stable in plasma and under applied pressure. However, at 2.5 MHz, their echogenicity was low. Another type of microbubble suspension has been obtained from sonicated albumin. Feinstein et al., J. Am. Coll. Cardiol., Vol. 11, pp. 59-65 (1988). Feinstein describes the preparation of microbubbles that are appropriately sized for transpulmonary passage with excellent stability in vitro. However, these microbubbles are short-lived in vivo, having a half-life on the order of a few seconds (which is approximately equal to one circulation pass) because of their instability under pressure. Gottlieb, S. et al., J. Am. Soc. Echo., Vol. 3, pp. 328 (1990), Abstract; and Shapiro, J. R. et al., J. Am. Coll. Cardiol., Vol. 16, pp. 1603-1607 (1990).
Gelatin-encapsulated microbubbles have also been described in WO 80/02365 by Rasor Associates, Inc. These are formed by “coalescing” the gelatin. Gas microbubbles encapsulated within a shell of a fluorine-containing material are described in WO 96/04018 by Molecular Biosystems, Inc.
Microbubbles stabilized by microcrystals of galactose (SHU 454 and SHU 508) have also been reported by Fritzch et al. Fritzsch, T. et al., Invest. Radiol. Vol. 23 (Suppl 1), pp. 302-305 (1988); and Fritzsch, T. et al., Invest. Radiol., Vol. 25 (Suppl 1), 160-161 (1990). The microbubbles last up to 15 minutes in vitro but less than 20 seconds in vivo. Rovai, D. et al., J. Am. Coll. Cardiol., Vol. 10, pp. 125-134 (1987); and Smith, M. et al., J. Am. Coll. Cardiol., Vol. 13, pp. 1622-1628 (1989). EP 398 935 by Schering Aktiengesellschaft discloses the preparation and use of microencapsulated gas or volatile liquids for ultrasound imaging, where the microcapsules are formed of synthetic polymers or polysaccharides. European Patent 458 745 by Sintetica discloses air or gas microballoons bounded by an interfacially deposited polymer membrane that can be dispersed in an aqueous carrier for injection into a host animal or for oral, rectal, or urethral administration, for therapeutic or diagnostic purposes.
WO 92/18164 by Delta Biotechnology Limited describes the preparation of microparticles by spray drying an aqueous protein solution to form hollow spheres having gas entrapped therein, for use in imaging. WO 93/25242 describes the synthesis of microparticles for ultrasonic imaging consisting of a gas contained within a shell of polycyanoacrylate or polyester. WO 92/21382 discloses the fabrication of microparticle contrast agents which include a covalently bonded matrix containing a gas, wherein the matrix is a carbohydrate. U.S. Pat. Nos. 5,334,381, 5,123,414 and 5,352,435 to Unger describe liposomes for use as ultrasound contrast agents, which include gases, gas precursors, such as a pH activated or photo-activated gaseous precursor, as well as other liquid or solid contrast enhancing agents.
Others have looked at the effect of the gas which is encapsulated, and suggested the use of fluorinated gases to enhance imaging as compared to air. U.S. Pat. No. 5,393,524 to Quay discloses the use of agents, including perfluorocarbons, for enhancing the contrast in an ultrasound image. The agents consist of small bubbles, or microbubbles, of selected gases, which exhibit long life spans in solution and are small enough to traverse the lungs, enabling their use in ultrasound imaging of the cardiovascular system and other vital organs. EP 554213 by Bracco discloses the use of fluorinated hydrocarbon gases to prevent collapse of microvesicles upon exposure to pressure in the bloodstream. WO 95/23615 by Nycomed discloses microcapsules for imaging which are formed by coacervation of a solution, for example, a protein solution, containing a perfluorocarbon. WO 95/03357 by Massachusetts Institute of Technology discloses microparticles formed of polyethylene glycol-poly(lactide-co-glycolide) block polymers having imaging agents encapsulated therein, including gases such as air and perfluorocarbons. As described in WO 94/16739 by Sonus Pharmaceuticals, Inc., while solids and liquids reflect sound to a similar degree, gases are known to be more efficient and are the preferred media for use as ultrasound contrast agents. In fact, as shown by Example 12 of WO 94/16739, protein microcapsules were dismissed as raising safety concerns (as well as efficacy issues) when administered to mini-pigs. U.S. Pat. Nos. 6,132,699 and 5,611,344 both describe methods of enhancing contrast using perfluorocarbon gases in synthetic polymeric shells. U.S. Pat. No. 5,837,221 describes a method of making a porous polymeric microparticle having a hydrophobic agent incorporated into the polymer to increase echogenicity.
Several ultrasound contrast agents have been approved in either the United States or Europe for very limited cardiac applications. OPTISON® (Amersham, Mallinkrodt) consists of heat denatured human albumin microcapsules containing the gas octafluoropropane. Each mL of microsphere suspension contains 5-8×108 microspheres with a mean diameter in the 2-4.5 micron size range and 220 μg octafluoropropane. These microspheres have not been approved for myocardial blood flow assessment and have only been approved for ventricular chamber enhancement. At high bolus doses (5 mL suspension or 1100 μg octafluoropropane), ventricular chamber enhancement lasts up to 5 minutes.
DEFINITY® (Bristol Myers Medical Imaging) consists of octafluoropropane containing lipid microspheres where the lipid shell is comprised of the phospholipids DPPA, DPPC, and mPEG-DPPE. Each mL of suspension contains 1.2×1010 microparticles having a mean diameter in the 1.1-3.3 micron size range and 1100 μg of octafluoropropane. The agent is only approved for ventricular chamber enhancement and not myocardial blood flow assessment. At a bolus dose of 700 μL (for a 70 kg person) or 5133 μg of gas, the agent has an enhancement duration in the ventricular chambers of approximately 3.4 minutes.
IMAGENT® (Photogen Inc.) consists of lipid microspheres containing pefluorohexane where the lipid shell is comprised of the phospholipid DMPC. Each mL of suspension contains 1.4×109 microparticles having a mean diameter less than 3 microns and 92 μg of perfluorohexane. The agent is only approved for ventricular chamber enhancement and not myocardial blood flow assessment. At a bolus dose of 0.43 mL (for a 70 kg person) or 40 μg of gas, the agent has a mean enhancement duration in the ventricular chambers of approximately 2.6 minutes.
In all cases, these commercial agents have limited utility and are not approved for applications other than ventricular chamber enhancement and provide mean image enhancement durations in the ventricular chambers lasting for periods of 5 minutes or less. There is a lack of commercial ultrasound contrast agents which allow enhanced images of the cardiovascular system, particularly of the myocardium and the ventricular chambers, for long duration. The agents described in the prior art when administered as a bolus or short infusion result in images of the myocardium which last for significantly less time than the amount of time required to conduct a complete examination of the heart. Typically, the prior art agents provide images that last for well below one minute for the myocardium. An agent that can provide enhanced image durations exceeding one minute in the myocardium and/or greater than 5 minutes in the ventricular chambers is desirable.
It is therefore an object of the invention to provide a dosage formulation containing microparticles that provides enhanced images and images of long duration, particularly for cardiac applications.
It is another object of the invention to provide a kit for administering the dosage formulation containing microparticles for use in ultrasound imaging techniques.